Sunlight Conversion Technologies

1.1.1 Photovoltaic Cell

Photovoltaic cells can convert sunlight directly into electricity. It has been recognized as one of the most sustainable clean energy technologies for generating electrical power on a large scale and has attracted worldwide attention. Few processes use photovoltaic cells to generate electricity through solar energy. First, sunlight is collected and induced into a charge-separated state to generate electron–hole pairs. The photon absorption process occurs in the silicon semiconductor film that causes the migration of excitons to a p-n junction in which charge separation occurs, producing an electromotive force. This p-n junction is connected to an external electrically circuit. A photo-current flows through the circuit, generating electrical power and completing the photo-to-electricity conversion process.^[3] The highest energy conversion efficiency of commercially available crystal silicon photovoltaics is approximately 18%. Apart from improving the efficiency of photo-electricity conversion, increased attention must be paid to the storage and distribution of solar energy for practical applications.

The most reasonable technology involves converting electrical energy into a form of chemical fuel, such as hydrogen or low-carbon organics, which can be easily stored and transported.

Although solar energy is clean and rich, the major challenge of current technology in making photovoltaic cells that can be competitive with fossil fuels is their high cost, which is two to five times higher than that of conventional energy

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technologies.^[4] The most used materials in photovoltaics are crystalline silicon wafers with thicknesses of around 180 μm to 300 μm. The production of crystalline silicon-based photovoltaic cell technology also requires high temperature, high pressure, and complex processes. Both the materials and their associated processing are expensive. To overcome the challenge of high material cost and to produce thin-film photovoltaic cells that can reduce the usage quantity of silicon-based materials, various type of technologies that increase the absorbance of visible light have been explored, including dye-sensitized solar cells and organic photovoltaics.^[5,6] These systems use relatively cheap materials and have higher potential for future utilization than crystalline silicon photovoltaic cells.

1.1.2 Thin-film Photovoltaic Cells

To reduce the cost of photovoltaic cells, thin-film photovoltaic cells should be explored because they use less expensive crystalline silicon-based photo-materials. Some thin-film solar cells with 1 μm- to 2 μm-thick photo-material film fabricated on cheap substrates, such as conductive glass, stainless steel, or plastic, have been investigated and validated. In terms of improving the efficiency of photo-electricity conversion, semiconductors used to harvest solar energy in thin-film photovoltaic cells are typically made of GaAs, CdTe, CuInSe2, and amorphous/polycrystalline silicon to substitute for expensive, conventional crystalline silicon.^[4]

1.1.3 Wet-chemical Photosynthesis

New sunlight conversion technology has been explored by learning from nature. Wet-chemical photosynthesis technology has been examined to achieve

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efficient and stable energy conversion, which operates under mild condition and uses only earth-abundant materials. In the long-term, photosynthesis technology should be able to generate solar fuels by simulating chemical reactions that occur naturally in biological systems. Photosynthesis is a wet-chemical process that transforms carbon dioxide and water into organic compounds, particularly sugars (chemical fuels), by using energy from sunlight. Harvesting solar energy, and then converting and storing it via chemical methods, similar to what occurs naturally in photosynthesis in green plants, is a possible strategy for meeting the challenges of solar energy applications.

1.1.4 Photoelectrolysis

Photoelectrolysis, which combines the concepts of photovoltaic cells and wet-chemical photosynthesis, uses photovoltaic cells to split water into hydrogen (chemical fuel), a process called solar water splitting. Photoelectrolysis can be regarded as an artificial form of photosynthesis. This process is assisted electrochemically: a solid photoelectrode is adopted to convert sunlight energy into splitting water and generating chemical energy. Hydrogen plays a critical role in the development of green energy because it is the ultimate clean energy and can be used in fuel cells. However, hydrogen is primarily formed by steam reforming, in which fossil fuels are consumed and carbon dioxide is generated.^[7] The chemical reaction of the cracking process may be represented as

(1-1) Photoelectrolysis can be utilized to accomplish water splitting and produce hydrogen and oxygen, a sustainable energy source without any byproducts that contain carbon. Nevertheless, the conversion efficiency of photoelectrolysis

4 2 2 2

CH H OheatH CO

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remains lower than that of photovoltaic cells, which is limited mainly by the low performance of photoelectrodes.

In 1972, Honda and Fujishima accomplished the earliest work on photoelectrolysis by using a photoelectrochemical cell that contains a photoanode and a photocathode; in this cell, the anodic/cathodic reactions in the splitting of water was conducted using TiO2 as the photoelectrode.^[8] They discovered that both electrons and holes were generated on the semiconductor photoanode (TiO2) when the UV illumination experienced radiation. Moreover, the excited photo-electrons went through the out circuit to reduce water and form hydrogen on a Pt counter-electrode as the holes oxidized water to form oxygen on the surface of the TiO2-based electrode, which was maintained at a certain electrode potential. The present investigation offers the possibility for a renewable, carbon-free source of high-quality hydrogen that can undergo processing, storage, and later usage through photoelectrochemical water splitting.